1. Field of the Invention
The present invention relates to a misfiring detection system for internal combustion engines, which can detect an engine misfiring by monitoring fluctuations in an angular velocity of an engine crankshaft. Specifically, the present invention is directed to a system capable of reliably detecting an engine misfire by means of a ring gear sensor for monitoring an angular velocity of an engine crankshaft.
2. Description of the Prior Art
Recently, there have been disclosed and developed misfiring detection systems for internal combustion engines. Such misfiring detection is required for optimal combustion control for an internal combustion engine. As is generally known, when a cylinder of the engine is misfiring, unburnt gas is exhausted and consequently harmful components in the exhaust gas are increased, thereby resulting in lowering of the engine output and increased pollution. Furthermore, if the combustion control is achieved in such a manner as to increase an intake air amount with a high step-response during infrequent engine misfiring, an idle speed of the engine becomes unstable and thereby results in hunting. Therefore, a precise misfiring detection is required to determine whether infrequent misfiring occurs, whether a particular cylinder is constantly misfiring, and what cylinder is misfiring. As set forth above, such a misfiring detection is advantageous to optimally control various combustion conditions, such as an ignition timing and an intake air amount. For example, Japanese Patent First Publication (Tokkai) Showa 57-188748 discloses an engine misfiring detection system for an automotive vehicle, which can detect misfiring occurring in a cylinder of the engine by monitoring fluctuations in an angular velocity of an engine crankshaft by means of an electromagnetic pick-up, even when the vehicle is in a running state. Such a conventional misfiring detection systems includes a rotary disc fixed on the crankshaft and employing diametrically opposing tabs radially projecting from an outer periphery thereof and two non-contact microswitches provided in different angular positions offset from each other by a predetermined relative phase angle, for example 90.degree., in such a manner as to be in close proximity to the tabs, respectively. The prior art misfiring detection system determines the engine misfiring on the basis of an angular velocity difference between angular velocities derived from pulse signals output by the non-contact microswitches. However, in the above mentioned conventional misfiring detection system employing the rotary disc fixed on the crankshaft, since tab installation points and non-contact microswitch installation positions are fixed, two measuring timings for starting and ending angular velocity measurement for an engine crankshaft achieved by the microswitches are also fixed.
As is well known, ignition timing for an internal combustion engine is generally varied in response to various engine operating conditions, such as engine load and engine speed. If the ignition timing is controlled in accordance with a minimum spark advance required for obtaining best torque in an engine operating state, the crank angle .theta..sub.PMAX corresponding to a maximum combustion pressure is essentially kept constant, as seen in a solid line (i) of FIG. 3 illustrating changes in a combustion pressure P in relation to a crank angle. The aforementioned minimum spark advance required for best engine torque will be hereinafter abbreviated as a "MBT". On the other hand, if the engine is misfiring, a waveform of the combustion pressure P remarkably fluctuates and the pressure P is rapidly lowered as shown in a broken line (iii) of FIG. 3. The combustion pressure drop causes fluctuations in the angular velocity of the crankshaft. As a result, engine misfiring is detected by monitoring the fluctuations in the angular velocity.
As shown in a dot and dash line (ii) of FIG. 3, if the ignition timing is retarded from the MBT in accordance with various controls for enhancing both of driving stability and riding comfort, such as a knocking control executed for avoiding an excessive knocking occurrence and a shift-shock suppressing control executed for reducing shift-shock occurring during shifting in an automatic transmission, the crank angle .theta..sub.PMAX is also varied towards a retardation crank angle position. Therefore, if the previously described measuring timings for starting and ending the angular velocity measurement are fixed as shown in a timing chart (a) of FIG. 3, the monitored angular velocities are affected by the varied waveform of the combustion pressure P. In this case, fluctuations in the angular velocity can be detected irrespective of misfiring occurrences. This causes an erroneous determination for engine misfiring. In this manner, reliability for misfiring detection may be lowered in the prior art misfiring detection systems. Furthermore, it is difficult to precisely monitor extremely slight fluctuation in the angular velocity of the crankshaft by means of the electromagnetic pick-up.
In order to precisely monitor the previously noted slight fluctuations of angular velocity, there has been proposed another type of conventional misfiring detection system wherein a combination of the above noted rotary disc fixed on the crankshaft and non-contact microswitches are replaced with a combination of a ring gear and a ring gear sensor provided in close proximity to a toothed portion formed on the outer periphery of the ring gear (flywheel gear teeth). The ring gear sensor generates pulse signals in proportion to the number of teeth of the ring gear passing therethrough. In such conventional ring gear type misfiring detection systems, a misfiring state is determined on the basis of fluctuations in a measuring time necessary to monitor a predetermined number of teeth passing through the ring gear sensor, so as to insure high accuracy for monitoring the angular velocity of the crankshaft. Unless the ring gear has a tooth defect, the ring gear sensor type misfiring detection system is superior to the rotary disc type misfiring detection system with regard to measuring accuracy, because the angular velocity measurement of the ring gear type misfiring detection system is achieved by utilizing the outermost peripheral toothed portion of the ring gear having an extremely large diameter. However, since the ring gear teeth engage with a drive-pinion of the starting motor for engine starting, there is a possibility of a ring gear tooth defect. If a tooth defect, such as a broken, bent or displaced tooth for example, occurs, the ring gear sensor outputs an erroneous pulse signal and, as a result, an engine misfiring may be erroneously determined. Furthermore, there is a possibility that the ring gear sensor is deactivated due to a defect in a wiring harness necessary for the ring gear sensor. If such a harness defect, as noted above, occurs, engine misfiring cannot be detected.